1. Field of the Invention
The invention relates generally to substrate handling systems. In particular, the invention relates to substrate handling systems involving linear motion from a load lock to a vacuum processing chamber.
2. Background Art
The fabrication of semiconductor integrated circuits is one of several technologies involving the use of vacuum processing chambers for processing wafers or other substrates in a high vacuum, often with noxious gases or with plasmas. Typical chamber pressures for deposition and etching steps range from around a torr for chemical vapor deposition to a millitorr and below for sputtering. Etching pressures typically are intermediate. Establishing very low pressures requires a long pump down from atmospheric pressure and possibly heating of the chamber surfaces to remove adsorbed gases. The pump down problem is exacerbated by the trend toward single-wafer processing reactors in which only a single wafer is processed at a time in the reactor. There are various types of integrated circuits, including both electrical, optical, and opto-electronic circuits and micro electromechanical systems (MEMS), in which a large number of very small devices are formed on a substrate. In most cases, a significant number of identical chips are fabricated on a single wafer or substrate and then separated from each other after fabrication. Particles present a problem for integrated because a single airborne particle deposited on the substrate may ruin an entire integrated circuit chip.
For these and other reasons, both in a production environment and even in research, it has become common practice to maintain the pressure within the processing reactor chamber at a pressure close to the processing pressure even while a substrate is being transferred into or out of the processing reactor chamber. High-volume semiconductor fabrication lines rely largely on platforms or integrated tools having a central transfer chamber arranged around a central axis. Slit valves are formed on the walls of the transfer chamber and selectively separate the transfer chamber from multiple processing chambers and from a load lock through which wafers are loaded into the system from cassettes originally held at atmospheric pressure. Each of the processing chambers and the load lock has its own vacuum pumping system. A robot driven by one or more shafts extending along the central axis and connected to them through magnetic coupling or other types of vacuum feedthroughs controls a wafer paddle through a frog-leg mechanism which can both rotate around the central axis and move into any of the processing chambers or the load lock thereby allowing wafers to be passed through the slit valves between the load lock and the processing chambers. Such a system allows the rapid transfer of wafers between chambers in which processing times for a single step arc typically less than a minute.
However, such an integrated tool is not always appropriate. The central transfer chamber is large, and it and the robot are expensive. Many applications, particularly in research and development, do not require the high throughput or multiple processing chambers available in integrated tools but would still benefit from a load lock. Production of some optical circuits and MEMS may require deposition times on the order of hours, and such substrates are often processed in relatively small numbers. Some high-value circuits are manufactured only in relatively small quantities.
Accordingly, several single-wafer load lock and wafer delivery systems have been proposed. For example, a magnetic manipulator described by Bryson et al. in U.S. Pat. No. 5,105,932 and commercially available from Transfer Engineering and Manufacturing, Inc. of Fremont, Calif. may be used with a single-wafer load lock coupled through a slit valve to a single processing reactor. With the slit valve closed, a wafer is manually loaded onto a paddle held by the manipulator in the load lock. Thereafter, the load lock is closed and pumped down, and the slit valve is opened. The manipulator includes a rod which holds the paddle on its end and which is magnetically coupled to a control lever on the outside so that the lever can move the paddle and its wafer from the load lock into the processing chamber. A lift pin assembly in the processing chamber removes the wafer from the paddle, which is then withdrawn, and the slit valve is closed for wafer processing.
This arrangement is reliable, rugged, not prone to generate particles, and relatively inexpensive. However, the arrangement requires a relatively long arm extending from the load lock chamber on its side opposite the processing chamber. The rod has a length including the distance between the wafer positions in the load lock and processing chambers, the wafer diameter, and the required length of the magnetic coupling and lever. Further, the relatively long rod protrudes into the clean room from which the wafers are loaded. Clean room space of the quality required for integrated circuit fabrication is extremely expensive to build and maintain. Therefore a need exists for a load lock and wafer delivery system that is relatively small, that is, has a small footprint especially within the clean room.
Bryson et al. in U.S. Published Patent Application 2001/0041268 A1 describes one type of single-wafer load lock involving a two-stage extendible two-track carriage. However, this approach has not gained commercial success because of its complexity and perceived deficiencies.